bpo-30860: Consolidate stateful runtime globals. (#3397)

* group the (stateful) runtime globals into various topical structs
* consolidate the topical structs under a single top-level _PyRuntimeState struct
* add a check-c-globals.py script that helps identify runtime globals

Other globals are excluded (see globals.txt and check-c-globals.py).

Objects/exceptions.c

@@ -6,6 +6,8 @@
 
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
+#include "internal/mem.h"
+#include "internal/pystate.h"
 #include "structmember.h"
 #include "osdefs.h"